Tappets and lifters are basically the same thing. The lobe of the cam slides across the face of these tappets to lift the tappet against the pressure of the valve spring. A cam follower is round like a bearing, and the lobe of the cam makes the cam follower roll like a bearing as it is actuated. There is a bit of voodoo involved with the cam design, depending on what type of tappet or cam follower is used. The cam lobe which interacts with a flat tappet should have rounded flanks, otherwise, if there was no rounded flanks, only flat sides, then the flat side of the cam can "slap" against the flat face of the tappet. Cams which interact with a cam follower can have flat flanks, since the cam follower is round and there are not two flat faces to "slap" together. On older cars, the tappets or lifters were solid. This meant that the old engines would get noisy as surfaces wore and the valve lash became greater, requiring that every so often you had to adjust the clearances in the valve train. Then some bright spark had an idea---Lets make the tappets two piece, like a mini hydraulic cylinder. Then we can bleed some of the engines pressurized oil thru galleries to the tappets, and no matter how much slack wears into the valve train, the two piece hydraulic tappets will extend to take up the slack, thus doin g away with the necessity to manually set the valves.